Rub the flank steak with half of the sea salt, black pepper, cumin, and smoked paprika.
Place the seasoned beef in a slow cooker with the diced onion, bell pepper, garlic, and tomato puree.
Cover and cook on low for 7 to 8 hours until the beef is tender enough to shred with two forks.
Combine the masarepa, water, and the remaining sea salt in a bowl, kneading until a smooth dough forms, and let it rest for 5 minutes.
Divide the dough into two balls and flatten into discs about half an inch thick.
Heat olive oil in a cast-iron skillet over medium heat and cook the arepas for 5-6 minutes per side until they develop a golden, toasted crust.
Split the warm arepas open and stuff them generously with the shredded beef mixture and fresh avocado slices.